Effect of modification by five different acids on pumice stone as natural and low-cost adsorbent for removal of humic acid from aqueous solutions ‐ Application of response surface methodology

Abstract

Pumice stone is a natural, low-cost, available adsorbent. This study aimed to investigate the efficacy of modified pumice with five different acids (acetic acid, hydrochloric acid, phosphoric acid, sulfuric acid, and nitric acid) for the removal of humic acid (HA) from aqueous solutions. Four variables were investigated: adsorbent dose (1–7 g/L), contact time (15–75 min), initial concentration (5–25 mg/L), and pH (3−11) by response surface methodology (RSM) and lab experiments. Results revealed that the amount of humic acid removal by the raw and modified adsorbents was increased by increasing the adsorbent dosage and contact time, whereas, in terms of initial concentration and pH variables, the opposite was observed. The pH and initial HA concentration had the maximum and minimum effects on the adsorption process, in all used acids, respectively. For all the adsorbents, the optimum efficiency was achieved in the following conditions: contact time = 75 min, pH = 3, pumice dosage = 7 g/L, HA initial concentration = 5 mg/L.Additionally, it was seen that the pumice modification could considerably increase the adsorbent efficiency. The comparative effectiveness of acids on the pumice adsorbent was in the order of H2SO4 > HNO3 > H3PO4 > HCOOH> HCl. Furthermore, the data fitting using different isotherm models and adsorption kinetics specified that the data were following Langmuir and Freundlich isotherms and pseudo-second-order kinetics. The data fitting using D-R isotherm similarly revealed that the HA adsorption process was physical on all the adsorbents. This study demonstrated that the acid-modified pumice could be utilized as an economic, naturally available adsorbent for the HA removal.